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Structural Determinants of Synaptobrevin 2 Function in Synaptic Vesicle Fusion

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Structural Determinants of Synaptobrevin 2 Function in Synaptic Vesicle Fusion 6668 • The Journal of Neuroscience, June 21, 2006 • 26(25):6668–6676 Cellular/Molecular Structural Determinants of Synaptobrevin 2 Function in Synaptic Vesicle Fusion Ferenc Dea´k,1,2 Ok-Ho Shin,1 Ege T. Kavalali,1,4 and Thomas C. Su¨dhof 1,2,3 1Center for Basic Neuroscience, 2Howard Hughes Medical Institute, Departments of 3Molecular Genetics and 4Physiology, The University of Texas Southwestern Medical Center, Dallas, Texas 75390-9111 Deletion of synaptobrevin/vesicle-associated membrane protein, the major synaptic vesicle soluble N-ethylmaleimide-sensitive factor attachmentproteinreceptor(R-SNARE),severelydecreasesbutdoesnotabolishspontaneousandevokedsynapticvesicleexocytosis.We now show that the closely related R-SNARE protein cellubrevin rescues synaptic transmission in synaptobrevin-deficient neurons but that deletion of both cellubrevin and synaptobrevin does not cause a more severe decrease in exocytosis than deletion of synaptobrevin alone. We then examined the structural requirements for synaptobrevin to function in exocytosis. We found that substituting glutamine for arginine in the zero-layer of the SNARE motif did not significantly impair synaptobrevin-dependent exocytosis, whereas insertion of 12 or 24 residues between the SNARE motif and transmembrane region abolished the ability of synaptobrevin to mediate Ca 2ϩ-evoked exocytosis. Surprisingly, however, synaptobrevin with the 12-residue but not the 24-residue insertion restored spontaneous release in synaptobrevin-deficient neurons. Our data suggest that synaptobrevin mediates Ca 2ϩ-triggered exocytosis by tight coupling of the SNARE motif to the transmembrane region and hence forcing the membranes into close proximity for fusion. Furthermore, the fusion reactions underlying evoked and spontaneous release differ mechanistically. Key words: exocytosis; endocytosis; synaptic vesicle recycling; SNARE; spontaneous fusion; hippocampus; synapse; whole-cell patch-clamp Introduction bled as four-helical bundles composed of four SNARE motifs (in Synapses release neurotransmitter when an action potential (AP) the case of synaptic SNARE complexes, a single SNARE motif induces Ca 2ϩ influx into nerve terminals, and Ca 2ϩ in turn trig- each from synaptobrevin and syntaxin and two from SNAP-25) gers synaptic vesicle exocytosis. However, quantal vesicular re- (Hanson et al., 1997). Assembly of SNARE complexes is thought lease that is electrically indistinguishable from AP-driven release to catalyze fusion by forcing membranes into close proximity also occurs spontaneously in all synapses; moreover, under ex- (Weber et al., 1998). Although SNARE complex formation is perimental conditions, release can be induced by hypertonicity. primarily fueled by hydrophobic interactions, SNARE complexes Although all of these forms of release require soluble contain a central “zero layer” composed of a hydrophilic electro- N-ethylmaleimide-sensitive factor attachment protein receptor static interaction mediated by three glutamine and one arginine (SNARE) proteins (for review, see Jahn et al., 2003), it is currently residue (Sutton et al., 1998). As a result, SNARE proteins are unclear whether the different forms of release are mechanistically classified based on their SNARE motif sequences into types of identical or are mediated by distinct types of fusion. Q-SNAREs and R-SNAREs (Fasshauer et al., 1998). During synaptic vesicle fusion, the synaptic vesicle SNARE Synaptobrevin 2 is a minimal SNARE that consists only of a protein synaptobrevin 2 (Syb2)/vesicle-associated membrane short NH2-terminal sequence, a SNARE motif, and a COOH- protein 2 (VAMP2) forms a complex with the plasma membrane terminal transmembrane region (see Fig. 1A). Initial evidence SNARE proteins syntaxin 1 and synaptosome-associated protein that synaptobrevin mediates fusion was obtained when synapto- of 25 kDa (SNAP-25) (Sollner et al., 1993). All SNARE proteins brevin was identified as the target of tetanus toxin, which blocks contain one or two SNARE motifs; SNARE complexes are assem- exocytosis by proteolytic cleavage of synaptobrevin (Link et al., 1992; Schiavo et al., 1992). Deletion of synaptobrevin impairs all Received Dec. 9, 2005; revised May 3, 2006; accepted May 3, 2006. forms of neurotransmitter release, albeit to different degrees This work was supported by the National Institutes of Mental Health Grant MH066198 (E.T.K) and the Howard (Schoch et al., 2001). Synaptobrevin-deficient synapses show an Hughes Medical Institute (F.D., T.C.S.). We are grateful to J. Pessin (Stony Brook, New York) for providing the ϳ100-fold reduction in AP-induced neurotransmitter release cellubrevinknock-outanimals.Wethank,fortheexcellenttechnicalassistance,AndreaRoth,IzaKornblum,andEwa ϳ Borowicz,andNickyHamlinandJasonMitchellforthehelpwithmousehusbandry.Wethankthehelpandadviceof but only a 10-fold reduction in spontaneous release or in re- Weiping Han, Peter Bronk, and Wen-Pin Chang with the lentivirus production. lease triggered by hypertonic sucrose. This disparity between CorrespondenceshouldbeaddressedtoDr.ThomasC.Su¨dhoforDr.EgeT.Kavalali,CenterforBasicNeuroscience, evoked and spontaneous release was also observed in neurons University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390-9111. E-mail: lacking SNAP-25 (Washbourne et al., 2002). Interestingly, in [email protected] and [email protected]. DOI:10.1523/JNEUROSCI.5272-05.2006 synaptobrevin-deficient chromaffin cells, release was only de- Copyright © 2006 Society for Neuroscience 0270-6474/06/266668-09$15.00/0 creased by ϳ50%, whereas double-mutant mice that lack both Dea´k et al. • Synaptobrevin Function in Synaptic Vesicle Fusion J. Neurosci., June 21, 2006 • 26(25):6668–6676 • 6669 synaptobrevin and cellubrevin, a closely related R-SNARE pro- Synaptic boutons were loaded with N-(3-triethylammoniumpropyl)-4- tein (McMahon et al., 1993), lacked all release (Borisovska et al., (4-(diethylamino)styryl)pyridinium dibromide (FM2-10) (400 ␮M; In- ϩ 2ϩ 2005). Thus, in chromaffin cells, synaptobrevin and cellubrevin vitrogen, Carlsbad, CA) by a 90 s exposure to 47 mM K /2 mM Ca ; full are functionally redundant. To some degree, redundancy was destaining was triggered with a hyperkalemic bath solution containing 90 mM KCl substituted for NaCl. Images were taken after 10 min wash in also reported for SNAP-25 and SNAP-23 function in chromaffin ϩ dye-free, nominally Ca 2 -free solution to minimize spontaneous re- cells (Sorensen et al., 2003). lease. In all experiments, isolated boutons (ϳ1 ␮m 2) were selected for Although synaptic SNARE proteins have been extensively analysis. Synaptic vesicle fusion was induced by gravity perfusion of hy- studied biochemically and physiologically, little is known about perkalemic solution onto the field of interest (1–2 ml/min) for 60 s, the structure/function relationships of synaptobrevin. For exam- followed by nominal Ca 2ϩ-free Tyrode’s perfusion for 60 s again. Images ple, the current model for how SNAREs catalyze fusion requires were acquired with a cooled CCD camera (Roper Scientific, Trenton, NJ) that the SNARE motif must be close to the transmembrane re- during illumination (1 Hz and 200 ms) at 480 Ϯ 20 nm (505 dichroic gion, but this has only been examined in yeast fusion reactions long pass and 534 Ϯ 25 bandpass) via an optical switch (Sutter Instru- (Wang et al., 2001) and in in vitro fusion assays (McNew et al., ments, Novato, CA) and analyzed using Metafluor Software (Universal Imaging, Downingtown, PA). Background staining levels determined 1999; Parlati et al., 2000; Melia et al., 2002). Therefore, in the ϩ present study, we used rescue experiments in cultured after five consecutive rounds of high K application were subtracted from all fluorescence images. synaptobrevin-deficient neurons to examine the structural re- Electrophysiology. Synaptic responses were recorded from pyramidal quirements for synaptobrevin function in synaptic exocytosis. cells in modified Tyrode’s bath solution in the whole-cell patch configu- ration. Data were acquired with an Axopatch 200B amplifier and Materials and Methods Clampex 8.0 software (Molecular Devices, Union City, CA), filtered at 2 ␮ Hippocampal cultures. Double knock-out (KO) mice were bred by cross- kHz, and sampled at 200 s. The internal pipette solution included the ing heterozygous mutant synaptobrevin 2 KO mice (Schoch et al., 2001) following (in mM): 115 Cs-MeSO3, 10 CsCl, 5 NaCl, 0.1 CaCl2,10 that are also homozygous mutant for cellubrevin (Yang et al., 2001). HEPES, 4 Cs-BAPTA, 20 tetraethylammonium-Cl, 4 Mg-ATP, 0.3 mM Hippocampal neurons from embryonic day 18 littermate mice were dis- Na2-GTP, and 10 lidocaine N-ethyl-bromide, pH 7.35 (300 mOsm). A sociated by trypsin (5 mg/ml for 5 min at 37°C), triturated with a sili- hypertonic solution, prepared by addition of 500 mM sucrose to the 2ϩ conized Pasteur pipette, and plated onto 12 mm coverslips coated with nominally Ca -free Tyrode’s solution, was perfused directly into the poly-L-lysine (approximately three coverslips per hippocampus). Neu- close vicinity of the cell from which the recording was made. Field stim- rons were cultured at 37°C in a humidified incubator with 95% air/5% ulations (24 mA pulses of 0.6 ms) were applied with parallel platinum electrodes immersed into the perfusion chamber. CO2 in Minimal Essential Media containing 5 g/liter glucose, 0.1 g/liter transferrin, 0.25 g/liter insulin, 0.3 g/liter glutamine, 5–10% heat- Coimmunoprecipitation
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